Proposed Supplement to PCC-2 2015 Repair of Pressure Equipment and Piping Tentative Subject to Revision or Withdrawal Specific Authorization Required for Reproduction or Quotation ASME Codes and Standards PART 2 REPAIR WELDING CONSIDERATIONS FOR Cr-Mo STEEL PRESSURE VESSELS ARTICLE 2.15 Guide for Selection of Repair Technique General wall thinning Local wall thinning Pitting Gouges Blisters Laminations Circumferential cracks Longitudinal cracks Cr-Mo Repair Y Y Y Y R R Y Y Nomenclature: Y= Generally appropriate S= Although it may be acceptable, is not generally used for this condition R= May be used, but requires special caution 1. INTRODUCTION 1.1 Scope Repair welding considerations in this article are applicable to pressure vessels for refinery, petrochemical, power generation and other services where the following requirements may be considered applicable. 1.2 Application (a) This article describes weld repair considerations for pressure vessels made from Cr-Mo steels. The purpose of this article is to provide the reader with a high level overview of deterioration mechanisms and subsequent considerations that need to be made in developing a detailed repair, examination and testing plan required for the successful repair of Cr-Mo pressurevessels. (b) The Cr-Mo materials listed in Table1 of this Article are susceptible to certain types of damage in elevated temperature service (e.g., see ref. 2, 3, and 4). (c) The repair of creep damaged Cr-Mo steels, creep enhanced ferritic steels, vanadium modified steels or stainless steel cladding or weld overlay are not included in this Article, but will be covered in a separate Article. (d) API RP 579-1/ASME FFS-1 and API RP 571 provide further information on temper embrittlement and other aging effects on the fracture toughness of Cr-Mo steels. 1.3 Design Temperature The maximum design temperatures of these materials are as listed in the applicable codes of construction. 1.4 Applicable Materials Typical generic Cr-Mo materials and their ASME designations are indicated in Table 1 however, equivalent international standard materials can also be used. Table1: Applicable Cr-Mo Steels Typical Materials 1¼Cr-½Mo 5Cr-½Mo SA387-12 Cl.1 SA387-12 Cl.2 SA387-11 Cl.1 SA387-11 Cl.2 SA387-22 Cl.1 SA387-22 Cl.2 SA387-21 Cl.1 SA387-21 Cl.2 SA387-5 Cl.1 SA387-5 Cl.2 9Cr-1Mo - ASME Designation Plates Forgings Vessel Piping Components SA182-F12 SA336-F12 SA182-F 11 SA336-F 11 SA182-F22 CL.1 & 3 SA336-F22 CL.1 & 3 SA182-F21 SA336-F21 CL.1 & 3 SA182-F5 SA182-F9 SA336-F9 SA335-P12 SA335-P11 SA335-P22 SA335-P21 SA335-P5 SA335-P9. SA542-B Cl.4 SA541-22 Cl.3 -
2. LIMITATIONS 2.1 Additional Requirements Part 1 of this Standard contains additional requirements. This Article shall be used in conjunction with Part 1 of PCC-2. 3. DESIGN 3.1 Feasibility Study of Repair Welding (a) The materials listed in Table 1 may be repair welded provided an investigation has been performed to determine the cause of the damage and provided appropriate weld repair procedures are used. (b) The structural integrity of the pressure vessel should be addressed prior to repairs together with the feasibility of the repairs and suitability of the pressure vessel for the intended service after the repairs. The serviceability or Fitness for Service Assessment should be based on API RP 579-1/ASME FFS-1 as shown in Figure.1. Figure 1: Standard Steps In Repair Welding 3.2 Evaluation of applicability of weld repair 3.2.1 Consideration of in-service degradation (a) In-service degradation, as shown in Table 2 and Figure 2, shall be considered before developing a repair welding procedure. (b) Typical considerations for in-service degradation for weld repair are shown in Table 3. (c) Further information on in-service degradation is provided in API RP 571 and in WRC Bulletins 488, 489 and 490. Type of Damage Applicable Operational Degradation Phenomena Typical Susceptible Temper Embrittlement Creep Embrittlement Hydrogen Attack Hydrogen Embrittlement Conditions 370-580ºC (700-1,080ºF) Over 454ºC (850 ºF) and with applied load High temperature and high pressure hydrogen environment High temperature, and high pressure hydrogen environment and start up and shut down conditions Toughness degradation in base metal and welds through the intergranular micro segregation of impurity elements as measured by the J factor for 2¼Cr & higher Cr base metals and X bar factor for weld metals as well as 1Cr & 1¼Cr base & weld metals Carbide precipitation and crack initiation in the coarse grain HAZ of a localized stressed area such as at a nozzle attachment weld Generation of methane bubbles, blisters and cracks See API 941 Toughness degradation by hydrogen absorption. Materials 1¼Cr-0.5Mo 5Cr-1Mo Embrittlement manifests at lower temperatures during start up and shutdown 1¼Cr- ½Mo Low Cr materials in high hydrogen partial pressure environment 1¼Cr-½Mo
Thermal Fatigue Large temperature Fracture crack propagation gradients during operation, start up and shutdown conditions Table 2: Typical In-Service Degradation All materials 3.3 Examples of Damage (a) Figure 2 indicates some examples of damage that can occur in Cr-Mo pressure vessels with or without stainless steel cladding or weld overlay. (b) The typical example shown is for high temperature high pressure (HTHP) pressure vessels in refining service. Figure 2: Examples of Damage Common to Cr-Mo Pressure Vessels Type of Damage Main Concerns Repair Considerations Temper embrittlement Low toughness at start up and shut down Operating temperature limits Weldability Creep embrittlement Detection by NDE Flaw removal Hydrogen attack Detection by NDE Flaw removal Hydrogen embrittlement Toughness at operating temp Weldability Notes: (Table includes prevention/mitigation for repair and /or replacement) De-embrittled heat treatment above 600ºC(1100ºF) then rapid cooling Use welding materials with low impurity levels Elimination of stress riser and higher Cr material selection Higher Cr material selection (ref. Nelson chart, API Std 941 Stainless steel weld overlay cladding De-hydrogenation heat treatment above 300ºC (570ºF) 1 hr min Low hydrogen welding process Table 3: Typical Considerations For Weld Repair For In-Service Degradation 3.4 Development of Weld Repair Procedures (a) The selection of weld repair method should be based on the reliability of the repaired area considering the future operation period as shown in Figure 3. (b) Sleeve repair and partial patch repair methods (refer to Table 4) are normally applied temporarily and are not recommended for periods beyond the next upcoming shutdown or outage without appropriate nondestructive examination (NDE) and applicable Fitness for Service Assessment.
. Figure 3: Flow Chart For The Selection Of Repair Welding Methods 3.5 Applicable Repair Welding Methods Some applicable repair welding approaches and alternates to post weld heat treatment that are outlined in ASME PCC-2 are listed in Table 4 with some additional limitations and considerations. 3.6 Welding / Preheat When the actual aged condition of the component to be repaired cannot be sufficiently evaluated for development of a repair welding procedure, a bead-on plate test* should be used to verify the repair welding procedure. * A bead-on plate test is a type of self-restraint weld test used to evaluate the cracking sensitivity of the base materials and arc welding consumables, See PVP 2011-57809 and 57079 References. Types of Repair Applicable Repair Remarks Methods in ASME PCC-2 Sleeve repair Article 2.6 Replacement with type B sleeve at the first Overlay welding and or internal weld metal build up Article 2.11 available opportunity is recommended In case of corrosion metal loss, welding materials shall be selected considering cause of corrosion. Butt-welded insert plates Article 2.1 Thickness of insert plate shall generally not be thicker than shell or head. Alternates To PWHT Article 2.9 Refer to paragraph 4.7 Alternates to traditional welding preheat Article 2.8 Welding strategies as indicated in PCC-2 Article 2.8 may provide permissible alternatives to preheat requirements Table 4: Applicable Repair Methods In PCC-2 4. FABRICATION 4.1 Weld Repair Procedures (a) Weld repair procedures may be developed as indicated in Table 5. (b) The Welding Procedure Specification (WPS) shall be qualified in accordance with ASME BPV Code Section IX as applicable and/or the requirements imposed by the code of construction. 4.2 Preparation for welding (a) For shielded metal arc (SMAW), drying of electrodes shall be carried out to minimize the potential for hydrogen cracking. (b) Welding bevel surfaces shall be clean, dry and free of oil, paint or other contaminants 4.3 Welding Conditions (a) In order to prevent hardening of welds, short length weld beads less than 2 inch (50 mm) bead length, should be avoided. (b) Special precaution shall be taken to guard against brittle fracture due to local thermal temperature gradients (c) For one side repair welding of piping, back shielding should be considered for and higher alloy steels. Seq Item Procedure Remarks
1 Identification of flaws Dimension and location using VT followed by NDE (PT, MT, & UT) Flaw size, distribution, location and depth 2 Removal of flaws By grinding or gouging Finish grinding is required 3 Examination of groove Using MT or PT Ensure complete removal of defects Preheating Mandatory for Cr-Mo steels Temperature measured on both sides at the preheated area. See PCC-2 Art. 2.8 Weld repair Refer to Table 3 WPS/PQR required 4 Welding materials Repair Equivalent or better grade of materials than those used during the original shop fabrication Low hydrogen type for SMAW & FCAW materials welding Welding process GTAW, SMAW or FCAW Interpass temperature and heat input shall be controlled Post heating Burner, electric resistance or Prevention of cold cracking induction heating Finishing Surface finishing by grinding Removal of stress risers 5 Examination Using MT,PT,UT & RT Covering neighboring area 6 Local PWHT As required by Code. See WRC 452 for additional guidelines It may be necessary to guard against harmful thermal gradients 7 Examination Using MT, PT, & Hardness checks Recheck for defects 8 Pressure test As required by applicable Codes Heat pressure retaining material before and during pressurization to prevent brittle fracture Note: MT: magnetic particle testing, VT : visual testing, PT: penetrant testing, RT: radiography, UT: ultrasonic testing Table 5: Repair Approach Sequence (d) The temper bead welding method may be considered after evaluation in some cases for low alloy welds when post weld heat treatment (PWHT) will not be carried out. See paragraph 4.7. 4.4 Preheating and Post Heating (a) In order to prevent hardening of welds and cold cracking, preheating, post heating and dehydrogenation heat treatment (DHT) shall be mandatory requirements unless the following paragraphs stipulate otherwise.(b) Typical preheating and welding interpass temperatures are indicated in Table 6. Steel, 1¼Cr-½Mo 5Cr-½Mo 9Cr-1Mo. P-No/Group 4-1 5A-1 5B-1 5C-1 Minimum Preheating Temp.ºC (ºF) 120 (250) 150 (300) 200 (390) 177 (350) Maximum Interpass Temperature 300 (600) 300 (600) 300 (600) 300 (600) Table 6: Typical Preheat and Interpass Temperatures 4.5 De-embrittlement Heat Treatment When the materials are severely embrittled, a de-embrittlement heat treatment operation may be used to recover toughness of material as shown in Table 7. 4.6 Dehydrogenation Heat Treatment (DHT). The preheat temperature should be maintained until PWHT or dehydrogeneration heat treatment is performed. When the materials are required to cool to ambient temperature after repair welding, dehydrogenation heating shall be carried out at a minimum of 300ºC (570ºF) for a minimum of 1 hr, or for a duration to be agreed between Purchaser and Fabricator in order to prevent cold cracking. 4.7 Post Weld Heat Treatment (a) PWHT should be performed when required per applicable construction code or standards. (b) Temper bead and other welding methods as detailed in Article 2.9 Alternates To Post Weld Heat Treatment may be applicable to some low chrome steels when corresponding WPS/PQR s are developed specifically for the welding repair considering welding position and welding circumstances. (c) Temper bead methods are usually not appropriate for and higher chrome materials used for hydrogen service because of the high weld metal and HAZ hardnesses generated by the welding process. (d) In case of local PWHT, the PWHT procedure shall be developed which shall also include the arrangement of thermocouples and insulation in order to minimize the thermal stresses generated during the PWHT operation. WRC 452 and AWS D10.10 provide guidelines for developing a PWHT plan with specific band widths (soak band, heated band and gradient control band) to ensure that thermal gradients are not harmful. Type of Degradation Materials and Services To Be Considered De-Embrittlement
Hydrogen attack All Cr-Mo steels at high temperature/pressure hydrogen services Creep embrittlement, 1¼Cr-½Mo at over Not applicable due to irreversible 480ºC (900 ºF ) phenomena Temper embrittlement, at 370-580ºC (700-1,080 ºF) Heating at not less than 600 ºC (1,120 ºF) Hydrogen embrittlement, at high temperature hydrogen services De-hydrogenation shut down operation or heat treatment at temperature not less than 300ºC (570ºF) Table 7: De-embrittlement Heat Treatment 5. EXAMINATION 5.1 Nondestructive Examination (a) NDE as indicated in Table 5 shall be considered at each step of repair welding work. The appropriate NDE procedure(s) for the applicable repair shall be selected to meet code requirements and provide the level of examination necessary for the repair. (b) NDE procedures shall be in accordance with ASME BPV Code Section V and applicable construction code/standards (c) NDE before repair welding of pressure boundary: (1) The entire area of the pressure vessel that is to be repair welded shall be examined by means of visual examination (VT) or other NDE methods as may be applicable.to ensure that the area is free of any defect harmful to the repair operation, which may include welding, PWHT and pressure testing. (2) The need for carrying out pressure testing after repairs as well as the pressure used in pressure testing shall be evaluated in consideration of service conditions. (d) NDE after weld repair and after pressure test. (1) Complete NDE shall be performed in an area that is at least the maximum of either 2T (where T is the thickness of material) or 100 mm (4) inches from the edge of the repair welded, preheated or post weld heat treated area in order to ensure the area is free of defects. (2) NDE of that area shall also be performed after any pressure test that may have been carried out. (e) Acoustic emission testing may also be an effective means of examination following completion of repairs. (f) Where possible, in service NDE monitoring during operation is recommended for the repaired areas. (g) In some instances, the use of NDE in lieu of pressure testing will be appropriate for repairs. Refer to PCC-2 Article 5.2 for Non Destructive Examinaton In Lieu Of Pressure Testing for Repairs and Alterations. (h) Follow up NDE after the pressure vessel is returned to service shall be performed based on fitness-for service assessment requirements or applicable ISI Codes. 6. PRESSURE TESTING (a) The requirement for the applicability of a pressure test subsequent to weld repairs shall be evaluated. (b) If a pressure test is determined to be required after the repair welding of pressure bearing parts is completed, the pressure vessel or vessel part should be pressure tested in accordance with the requirements of the applicable construction Code. In lieu of the construction code pressure test requirements, PCC-2 Article 5.1 should be followed. (c) The pressure test, when required, shall be performed at a temperature higher than the Facture Appearance Transition Temperature (FATT) and at or above the minimum temperature specified by the code of construction.in order to prevent brittle fracture during the pressure test. (d) The toughness value of degraded materials shall be evaluated based on accumulated material database or samples obtained from vessel parts. (e) For pressure vessels that are to be hydrotested that operate in hydrogen service, the hydrotest pressure shall be evaluated in consideration of hydrogen service conditions and shall be no higher than the vessel operating pressure. (f) When a pressure test is to be carried out, consideration shall be given to the pressure train that the pressure vessel may be located in, the possibility of isolation of components within that train and consideration of the need for pressure testing the entire train 7 REFERENCES API RP571, 2010 Damage Mechanisms Affecting Fixed Equipment in The Refining Industry, published by API, Washington, D.C. WRC Bulletin 488, Damage Mechanisms Affecting Fixed Equipment in The Pulp and Paper Industry, published by the Welding Research Council, New York, NY. WRC Bulletin 489, Damage Mechanisms Affecting Fixed Equipment in The Refining Industry, published by the Welding Research Council, New York, NY. WRC Bulletin 490, Damage Mechanisms Affecting Fixed Equipment in Fossil Electric Power Industry, published by the Welding Research Council, New York, NY. API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry, published by API, Washington, D.C.
API RP 579-1/ASME FFS-1, 2007 Fitness-For-Service, published by API, Washington, D.C. ASME PCC-3, Inspection Planning Using Risk-Based Methods published by ASME, New York, NY. PVP2011-57809, Guideline for Repair Welding of Pressure Equipment in Refineries and Chemical Plants, Part 1 General, published by ASME, New York, NY. PVP2011-57079, Guideline for Repair Welding of Pressure Equipment in Refineries and Chemical Plants, Part 3 Carbon Steel, High Tensile Steel and Cr-Mo Steels, published by ASME, New York, NY. PVP2013-97227, Chronological Review Of Manufacturing Technologies and Considerations of Maintenance/Inspection For Heavy Wall Hydroprocessing Reactors, published by ASME, New York, NY. BS EN ISO 17642-2, 2005 Destructive test on welds in metallic materials- Cold cracking tests for weldments- Arc welding process- Self-restraint tests-part 2: Self-restraint tests, published by the British Standards Association, London, UK. WELDING JOURNAL, April 2002, Welding Process Effects in Weldability Testing of Steels, authored by Atkins, D; Thiessen, D; Nissley, N; Adonyi, Y; Published by the AWS, Miami FL. API RP 934 series, published by API, Washington, D.C. (a) 934-A, Recommended Practice for Materials and Fabrication Requirements for, -¼V, 3Cr- 1Mo & -¼V Steel Heavy Wall Pressure Vessels for High Temperature, High Pressure Hydrogen Service (b) 934-C, Recommended Practice for Materials and Fabrication Requirements for 1¼Cr-1/2Mo Steel Heavy Wall Pressure Vessels for High Pressure Hydrogen Service Operating at or below 825ºF (441ºC) (c) 934-D, Technical Report on the Materials and Fabrication Issues of 1¼Cr-½Mo and 1Cr-½ Mo Steel Pressure Vessels (d) 934-E, Recommended Practice for Materials and Fabrication Requirements for 1¼Cr-1/2Mo Steel Pressure Vessels for Service above 825ºF (440ºC) ASME BPV Code Section IX, Welding and Brazing Qualifications, published by ASME, New York, NY. ASME BPV Code Section V, Non Destructive Examination, published by ASME, New York, NY. WRC Bulletin 452, Recommended Practices for Local Heating of Welds in Pressure vessels, published by the Welding Research Council, New York, NY AWS D10.10, Local Heating of Piping and Tubing, published by AWS, Miami, FL